Theoretical study of the molecular structure and normal coordinate analysis of hydrogen cyanide addition compound with boron trifluoride, HCN-BF3

An extensive HF, MP2, B3LYP and CCSD study of the molecular structure and normal vibrations have been performed for the HCN-BF3 molecule. Calculations with a wide range of basis sets were classified into two groups based on the optimized N-B bond distance. The results for Group A are compared with the experimental structure of the solid phase molecules. The N-B lengths of Group A are approximately linear related to the N-B-F valence angles and also to the N-B stretching frequencies. HF/DZV calculation was used to represent the solid phase model. The N-B lengths of Group B are close to those of the gas phase molecule and both N-B-F angles and N-B sensitive frequencies have roughly the same values. Differences in the chemical bond between gaseous and solid phase HCN-BF3 are discussed based on the calculated force constants, vibrational frequencies and potential energy distributions. Vibration mode analysis indicates that the ν4 mode in the 600-700 cm−1 region can be assigned to the BF3 symmetric deformation, which shifts upon 10B/11B isotopic substitution. The ν5 mode which is insensitive to isotope substitution and changes band position with the N-B distance is assigned to the N-B bond stretching vibration.